To show how long viruses and worms live and spread on the internet, Kaspersky Labs this week noted that its monthly Top 20 report for September was the first list of this year that didn't contain malicious code from 2003.

"The old steadfasts Swen and Sobig have finally disappeared from the Top 20," said Olga Kobzareva, a researcher at Kaspersky, in an email to TechWeb.

"In other words, September finally saw malware created in previous years vanish totally from the ratings: we now have only viruses created in 2004 [on the list]."

Other tidbits gleaned from the Moscow-based security firm's Top 20 include the dominance of just a few worm "families" and the prevalence of foreign-made malware.

Sixteen of the top 20 worms and viruses in September came from the family trees of Netsky, Bagle, and MyDoom.

Together, those three clans accounted for 71 percent of all malicious code that Kaspersky detected during the month.

The bulk of the most pernicious worms have foreign origins, according to Kaspersky's list.

Netsky was reportedly created by a teenager now under indictment in Germany, while Bagle is thought to come from a rival hacker group based in Russia. And the Zafi.b worm, which holds down the number three spot at nearly 13 percent, claims Hungary as home.